My red button opens the ATC window.
90% of the time, ATC command 1 is what you need. Of that other remaining 10%, at least 8 of that will be ATC commands 2-5.
So I have the “trigger” as ATC 1. The up / down rockers (individually - buttons 5-8) are ATC commands 2-5. Using both the rockers is the default aileron trim (ironically, a SHIFT command by default as per the topic of this thread). And the red button opens the ATC panel so I can see what command I need to send, since I fly with the panel closed normally. That way I can basically do 98% of my ATC calls without having to take my left hand off my yoke.
And I have my XBox controller mounted to the middle of my yoke as per the link below. The D-Pad is ATC 6-9, and X is ATC 0.
And, I use a shift command there as well. LB + the D pad moves me around the cockpit like the arrow keys on the keyboard.